Method for manufacturing a polarizing film or a so-called polarizer (hereinafter, referred as “a polarizing film”) comprising a polyvinyl alcohol type resin (hereinafter, referred as “PVA type resin”) layer having a dichroic material impregnated therein in a molecularly oriented state through a dyeing process has been well known. Such known technologies include a method wherein a laminate including a thin PVA type resin layer formed by coating and drying a solution of polyvinyl alcohol type resin on a resin substrate is subjected to a dry stretching process using a stretching apparatus in a heating device such as an oven, and then subjecting the laminate to a dyeing process to thereby form a thin polarizing film having a dichroic material impregnated therein in a molecularly oriented state, and alternatively, a method wherein a laminate including a thin PVA type resin layer is first subjected to a dyeing process to have a dichroic material impregnated therein, and then subjected to a dry stretching process using a stretching apparatus in a heating device to thereby form a thin polarizing film having the dichroic material impregnated therein in a molecularly oriented state, as disclosed in the Comparative Test Sample in FIG. 2.
A polarizing film used for a liquid-crystal display element laminated on each of a front and back surfaces of a liquid-crystal cell is a polarizing film having a thickness of 20 to 30 μm having a dichroic material impregnated therein, manufactured by transporting a mono-layer of a PVA type resin generally having a thickness of 60 to 80 μm through a roll type transporting apparatus having a plurality sets of rolls driven at different peripheral speeds, and dyeing the mono-layer to have the dichroic material impregnated therein, and then wet-stretching the mono-layer using a solution under a temperature ranging from a room temperature to that around 60° C. This produces a thick polarizing film. Presently, polarizing films which are in practical use in large-sized screen televisions are those having high optical properties of a single layer transmittance of 42% or higher and a polarization rate of 99.95% or higher, and may be referred as thick high-performance polarizing films.
However, since the PVA type resin is hydrophilic, a polarizing film is sensitive to changes in temperature and humidity and is apt to produce changes in dimensions such as expansion or shrinkage possibly resulting in curls. Thus, in order to suppress expansion and shrinkage and to minimize influence of temperature and/or humidity, it has been a usual practice to provide a polarizing film having a protection film laminated on each of the opposite surfaces thereof. It should however be noted that, in the case of a thick polarizing film, it is still difficult to restrict the expansion or shrinkage of the polarizing film, thus, when the polarizing film is laminated to a component such as a liquid-crystal cell, a stress is induced in such component, and may cause a distortion in a displayed image in the liquid-crystal display element. The above is the technical problem inherent in the thick polarizing film. Additionally, it is needless to mention that the thick polarizing film has to satisfy up-to-date demands to reduce thickness and energy consumption.
In order to challenge the aforementioned problems, there have been needs to a new technology for providing thin polarizing films in place of the thick polarizing films. However, if a thin PVA type resin mono-layer, for example, a PVA type resin film having a thickness less than 50 μm is passed through a roll type transporting apparatus and subjected to a wet stretching process using a solution from a room temperature up to around 60° C., the film may dissolve in the solution, or may break because of being unable to withstand the tension because the thin PVA type resin film has a hydrophilic polymer composition. As such, it has been difficult to stably manufacture a thin polarizing film from a thin PVA type resin film. To address this, the methods disclosed in the Patent Documents 1 to 3 have been developed as new manufacturing methods of thin polarizing films, where a thin polarizing film is manufactured by forming a thin layer of a PVA type resin on a resin substrate of a certain thickness and stretching the formed thin PVA type resin layer together with the resin substrate.
The above methods are not of a wet type wherein a stretching is carried out in an aqueous solution, but of a type wherein a laminate film including a resin substrate and a PVA type resin layer is subjected to a dry stretching process so that the PVA type resin is stretched together with the resin substrate using a stretching apparatus in a heating device such as an oven, and then the laminate is immersed in a dyeing solution to thereby produce a thin polarizing film formed on a resin substrate and having a dichroic material impregnated therein in a molecularly oriented state. Such manufacturing method may allow for manufacturing a thin polarizing film of a few micrometer thick having the dichroic material impregnated therein in a molecularly oriented state by forming a PVA type resin layer having a thickness of ten micrometer or a little thicker by coating and drying a solution containing a PVA type resin to a resin substrate, then subjecting the PVA type resin layer to a dry stretching using a stretching apparatus in a heating device such as an oven, and dyeing the layer to impregnate a dichroic material therein.
This produces a thin polarizing film. The aforementioned manufacturing method and thin polarizing film are considered to be promising from a viewpoint of reducing the thickness of a display element, eliminating distortions in displayed images and allowing low energy consumption. However, up to now, the thin polarizing film manufactured by the aforementioned method has not overcome a technical problem of having a lower degree of optical properties as shown in the Comparative Test Samples 1 and 2 in FIGS. 4 and 5.
First, it is necessary to understand optical properties as background art. The optical properties of a polarizing film which can be used for a large size display element is represented in short by a polarization rate P and single layer transmittance T. The performance of the polarizing film is shown by a T-P graph which includes plotted values of the two optical factors such as the polarization rate P and the single layer transmittance T which are in a trade-off relationship.
Reference is now made to the diagram in FIG. 6. It is to be noted that an ideal property is a case where T is 50% and P is 100%. Note that it is easier to increase the value of P in a range where the value of T is relatively low, and it is difficult to increase the value of P in a range where the value of T is relatively high. Thus, it is considered that the polarization rate P of 99.95% or higher and the single layer transmittance T of 42.0% or higher are, currently or even in future, optical properties required for the performance of polarizing film for a large size display element since those specific values have not been realized in a thin polarizing film. The ideal property is a case where T is 50% and P is 100%, but when light transmits through a polarizing film, a part of light is reflected at a boundary between the polarizing film and air. Taking such reflection into consideration, it is noted that the transmittance is reduced by an extent corresponding to the portion of the reflected light, and the maximum attainable value of the single layer transmittance T may be around 45 to 46%.
The polarization rate P may be considered as representing a contrast ratio (CR) of a polarizing film or a display. The polarization rate P of 99.95% corresponds to the contrast ratio (CR) of 2000:1 of a polarizing film, and when this polarizing film is used in a cell for a liquid-crystal television commercially available, the displayed image contrast ratio (CR) corresponds to 1050:1. The contrast ratio (CR) will become better and easier to observe the displayed image with an increase in the contrast ratio (CR) of the polarizing film and that of the display. As will be described later, the contrast ratio (CR) of a polarizing film is a value of a parallel transmittance divided by a cross transmittance. The contrast ratio (CR) of a display is a value of the maximum intensity of brightness divided by the minimum brightness. The minimum brightness is the one in a black screen, and in the case of a liquid-crystal display television under a typical viewing environment, 0.5 cd/m2 or lower is required. With the minimum brightness higher than the value, there will be a reduction in the color reproducibility of the liquid-crystal display. The maximum brightness is the one under a display of a white screen, and a display with the maximum brightness or luminance in a range of 450 to 550 cd/m2 is used for a liquid-crystal display television under a typical viewing environment. With the maximum brightness or luminance lower than the value, there will be a reduction in visibility of the liquid crystal display.
It is considered from the above that, generally, the contrast ratio (CR) of 1000:1 or higher is required for a liquid crystal television. On the other hand, considering the depolarization in a liquid crystal cell, the contrast ratio (CR) of 2000:1 or higher is required for a polarizing film. This corresponds to the polarization rate P of 99.95 or higher.
In addition, a polarizing film having a single layer transmittance T of 42.0% or higher is generally used for liquid-crystal televisions. With the single layer transmittance T lower than the value, luminance L of the display may be reduced. For example, if the display luminance L is represented by 100 for a polarizing film having a single layer transmittance T of 42.0%, the display luminance L becomes 90 for a polarizing film having a single layer transmittance T of 40.0%. This means that in order to keep the display luminance L of 100 as in the case of a single layer transmittance T of 42.0%, it is necessary to increase a light source and/or lighting energy of a display using the polarizing film having the single layer transmittance T of 40.0% by 10%. Referring to a light source used for a display element, to make a display corresponding to the polarizing film having the single layer transmittance T of 42.0%, the display luminance L should be increased by making the light source itself to have brighter luminance.
The prior art documents referred to in the above and in the following descriptions are listed below.    Patent Document 1: Japanese Laid-Open Patent Publication JP2001-343521A    Patent Document 2: Japanese Patent 4279944B    Patent Document 3: Japanese Laid-Open Patent Publication JP51-069644A